Abstract
SummaryDespite its importance for membrane stability and pathogenicity of mammalian pathogens, functions of the O‐polysaccharide (OPS) of lipopolysaccharide (LPS) remain unclear in plant‐associated bacteria. Genetic information about OPS biosynthesis in these bacteria is largely missing. Genome analysis of various plant‐associated Pseudomonas strains revealed that one of the two known OPS biosynthesis clusters from Pseudomonas aeruginosa PAO1, the common polysaccharide antigen (CPA) gene cluster, is only conserved in some strains of the Pseudomonas fluorescens group. For the O‐specific antigen (OSA) biosynthesis cluster, the putative genomic position could be identified, but orthologues of most functional important OSA biosynthesis enzymes could not be detected. Nevertheless, orthologues of the glycosyltransferase WbpL, required for initiation of CPA and OSA synthesis in P. aeruginosa PAO1, could be identified in the analysed Pseudomonas genomes. Knockout mutations of wbpL orthologues in Pseudomonas syringae pv. tomato DC3000 (Pst) and Pseudomonas cichorii ATCC10857/DSM50259 (Pci) resulted in strains lacking the OPS. Infection experiments of Arabidopsis thaliana plants revealed a reduced entry into the leaf apoplast after spray inoculation and a reduced apoplastic amplification of Pst ∆wbpL. Stab and spray inoculation of lettuce (Lactuca sativa) leaves with Pci ∆wbpL causes reduced infection symptoms compared to the wild‐type strain. Furthermore, swarming motility was reduced in ∆wbpL mutants of Pst and Pci. This might be a possible reason for reduced bacterial titres after surface inoculation and reduced bacterial amplification in the plant. Our results imply that the presence of lipopolysaccharide OPS is required for efficient host colonization and full virulence of plant‐pathogenic Pseudomonas bacteria.
Highlights
The genus Pseudomonas comprises several plant-associated bacteria, including versatile plant pathogens that infect crops and cause substantial economic damage (Vanneste, 2017; Young, 2010)
Except for the P. aeruginosa strains, P. fluorescens Pf0-1, P. chlororaphis ATCC17415 and P. brassicacearum NFM421, the overall results of the comparative analysis indicate that the common polysaccharide antigen (CPA) cluster is not conserved in all plant-associated Pseudomonas species
In accordance with the lack of complementation of the LPS phenotype, transient expression of the Pst or Pseudomonas cichorii ATCC10857/DSM50259 (Pci) wbpL gene in Pst ∆wbpL did not reconstitute the motility phenotype. These results show that a loss of the OPS diminishes swarming motility in both bacterial strains
Summary
Despite its importance for membrane stability and pathogenicity of mammalian pathogens, functions of the O-polysaccharide (OPS) of lipopolysaccharide (LPS) remain unclear in plant-associated bacteria. Genome analysis of various plant-associated Pseudomonas strains revealed that one of the two known OPS biosynthesis clusters from Pseudomonas aeruginosa PAO1, the common polysaccharide antigen (CPA) gene cluster, is only conserved in some strains of the Pseudomonas fluorescens group. Orthologues of the glycosyltransferase WbpL, required for initiation of CPA and OSA synthesis in P. aeruginosa PAO1, could be identified in the analysed Pseudomonas genomes. Infection experiments of Arabidopsis thaliana plants revealed a reduced entry into the leaf apoplast after spray inoculation and a reduced apoplastic amplification of Pst ∆wbpL. Our results imply that the presence of lipopolysaccharide OPS is required for efficient host colonization and full virulence of plant-pathogenic Pseudomonas bacteria
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